7.1 Feeder Conductor Sizing
Key Takeaways
- A feeder is a circuit conductor between the service equipment / source of a separately derived system / power-supply source and the final branch-circuit overcurrent device (Art. 100) — after the service disconnect you are in feeder territory, not Article 230 service rules
- NEC 215.2(A)(1) requires feeder conductors to have ampacity not less than the noncontinuous load plus 125% of the continuous load (unless assembly ratings permit otherwise under the section’s conditions)
- Select feeder ampacity from Table 310.16 (or other applicable ampacity tables) after applying 310.15 adjustment/correction, and respect 110.14(C) termination temperature limits
- Article 220 calculated feeder/service loads drive the required ampacity; do not size a feeder from the panelboard bus rating alone or from the upstream breaker frame size without reading the load
- Neutral (grounded) feeder conductors follow 220.61 demand and 215.2 sizing rules — a reduced neutral is permitted only where the Code calculation supports it
Why Feeder Sizing Matters on the Arkansas JW Exam
The Arkansas Journeyman Electrician exam (Prov / ADLL, NEC 2023, 60 questions, 3 hours, 70% pass) weights Service, Feeders, Branch Feeders and Overcurrent Protection at 8 items. Chapter 6 covered Article 230 services. This chapter starts on the load side of the service disconnect, where conductors become feeders and overcurrent rules shift into Articles 215 and 240.
On an open-book form you will lose time if you open 230.42 when the stem clearly says “feeder,” or if you size conductors from breaker frame folklore instead of 215.2 + Table 310.16. Tab Article 100 (Feeder), 215.2, 220 (calculated load inputs), 110.14(C), and Table 310.16 as a cluster before exam day.
Feeder Identity — Article 100
Feeder (Art. 100): All circuit conductors between the service equipment, the source of a separately derived system, or other power-supply source and the final branch-circuit overcurrent device.
Practical exam checkpoints:
| Location on the one-line | Usually called |
|---|---|
| Utility → service disconnect | Service conductors (Art. 230) |
| Service disconnect → subpanel / distribution panel | Feeder (Art. 215) |
| Final OCPD → outlets/utilization equipment | Branch circuit (Art. 210) |
| Generator / SDS → first OCPD / distribution | Often feeder or SDS supply — read the stem |
Prov trap: A long run from a main panel to a garage subpanel is a feeder, even if someone calls it a “branch” in the field. Once you pass the service disconnect, stop applying service-entrance clearance and drip-loop rules unless the stem still asks about the service side.
Minimum Feeder Ampacity — 215.2
215.2(A)(1) is the core sizing rule for feeder conductors serving any load:
- Ampacity shall not be less than the noncontinuous load plus 125% of continuous load.
- Where the assembly (including OCPDs) is listed for operation at 100% of its rating, the section’s listed-assembly path may allow different continuous-load treatment — open the exception language when the stem invokes 100%-rated equipment.
Continuous load (Art. 100): A load where the maximum current is expected to continue for 3 hours or more. On Prov stems, lighting in stores, office receptacles treated as continuous by design notes, and process loads marked “continuous” are flags for the 125% adder.
Worked pattern (verify numbers in your book)
Suppose a feeder supplies 40 A continuous + 20 A noncontinuous:
- Required ampacity ≥ 40 × 1.25 + 20 = 70 A.
- Choose a conductor whose allowable ampacity (after adjustment/correction) is at least 70 A under the termination temperature column you are allowed to use.
- Then select the OCPD under Article 240 (next section) — conductor ampacity and breaker rating are related but not identical questions.
Do not multiply the entire mixed load by 125% unless every ampere is continuous. The Code formula is 125% of continuous + 100% of noncontinuous.
Connecting Article 220 Calculations to Feeders
Article 220 produces the calculated load that 215.2 must carry. Exam workflow:
- Perform (or accept from the stem) the Article 220 feeder/service calculation — general lighting, receptacles, appliances, HVAC, demand factors, etc.
- Separate continuous vs noncontinuous portions when the stem or occupancy rules require it.
- Apply 215.2 minimum ampacity.
- Select conductors from Table 310.16 (copper or aluminum as stated).
- Apply 310.15 ambient correction and bundling adjustment when the stem gives those facts.
- Confirm termination temperature under 110.14(C) before claiming a 90°C column ampacity for equipment rated 60°C or 75°C.
Trap: Using the 90°C column of Table 310.16 for THHN terminations on standard 75°C-rated breakers/lugs is a common wrong answer. Ampacity for sizing to equipment is generally limited to the 60°C or 75°C column as applicable under 110.14(C), even when the insulation is rated 90°C (90°C ampacity still matters for adjustment/correction starting points where the Code permits).
Feeder Conductor Selection Checklist
Use this sequence on timed items:
- Confirm it is a feeder — load side of service / SDS / source, ending at a final branch OCPD.
- Gather load — continuous amperes, noncontinuous amperes, or a 220 calculated load.
- Compute minimum ampacity — 215.2(A)(1) (and any dwelling or specific feeder rules the stem cites).
- Pick material and insulation — Cu vs Al; insulation type for temperature rating.
- Read Table 310.16 at the correct temperature column.
- Adjust for ambient and more than three current-carrying conductors if given.
- Compare adjusted ampacity to the 215.2 minimum; upsize if needed.
- Neutral — size grounded conductor per 215.2 / 220.61; do not automatically make the neutral the same size as ungrounded conductors unless the calculation requires it or a specific rule says so.
Grounded (Neutral) Feeder Conductors
215.2 and 220.61 together govern feeder neutrals:
- The grounded conductor must be adequate for the maximum unbalanced load.
- Demand factors in 220.61 may reduce the neutral load for certain dwelling and feeder conditions — open the section when the stem asks for minimum neutral size on a feeder.
- Nonlinear loads and certain multiwire arrangements can force a full-size or larger neutral consideration — if the stem emphasizes harmonics or heavy electronic loads, do not casually downsize.
Exam skill: a question about “minimum size of the grounded conductor of a feeder” is often a 220.61 lookup, not a copy of the ungrounded conductor size from Table 310.16 alone.
Feeders to Separate Buildings and Outside Circuits
When a feeder supplies a separate building or structure, Article 225 (Outside Branch Circuits and Feeders) adds disconnect, support, and clearance rules. Ampacity still starts with 215.2, but the stem may also ask for:
- A readily accessible disconnecting means for the building supplied (225.31 and related).
- Identification and grouping similar in spirit to service rules, but do not answer with Article 230 section numbers when the one-line shows a feeder from Building A to Building B.
If the stem says “feeder to a detached garage from the dwelling service panel,” you are in 215 + 225, not service-entrance drip loops.
Parallel Feeders and Large Ampacities
For large feeders, 310.10(G) / parallel conductor rules require identical length, material, size, insulation, and termination manner when conductors are paralleled. Prov may ask whether paralleling is permitted under a given size threshold — open the parallel rules rather than inventing “always OK if the ampacity adds up.”
Also remember: the equipment grounding conductor for a feeder is sized from 250.122 based on the OCPD ahead of the feeder, not from 250.66 (GEC table). Mixing GEC and EGC tables is a frequent wrong-path error when feeder stems mention grounding conductors.
Timed Open-Book Tips for §7.1
- First word check: service vs feeder vs branch.
- Ampacity formula: 125% continuous + 100% noncontinuous under 215.2(A)(1).
- Table lookup: material + temperature column + adjustments.
- Neutral: open 220.61 when unbalanced load is the ask.
- Outside building: add Article 225, keep ampacity in 215.
- Never size the feeder from the subpanel bus label alone if the calculated load is larger — the conductors must meet the load rules.
Per the NEC definition, conductors between the service disconnecting means and a subpanel’s main breaker are generally classified as which of the following?
Under NEC 215.2(A)(1), what is the general minimum ampacity basis for feeder conductors serving continuous and noncontinuous loads (absent a listed 100%-rated assembly exception)?
A feeder supplies 48 A of continuous load and 16 A of noncontinuous load. Ignoring adjustment/correction, what minimum ampacity must the feeder conductors provide under 215.2(A)(1)?
When selecting feeder conductor ampacity from Table 310.16 for terminations on standard 75°C-rated equipment, which limitation most often controls the ampacity used for sizing?